1. Field of the Invention
The present invention relates to novel polymers based on boron and nitrogen and to a process for the preparation thereof.
This invention also relates to the use of such novel polymers in the production of ceramic materials and shaped articles based on boron nitride, especially boron nitride in fibrous form.
2. Description of the Prior Art
Boron nitride is increasingly in demand in this art in light of its high thermal stability, its resistance to thermal shock, its great chemical inertness and its very good thermal conductivity. On the other hand, its low electrical conductivity makes it an insulator of choice.
Several processes are presently known to the art for the preparation of boron nitride.
One such process includes reacting boron trichloride with ammonia in the gaseous state. A fine boron nitride powder is obtained in this manner, which may be sintered to produce solid shaped articles. However, the shaped articles thus produced exhibit a characteristic microporosity which may be highly detrimental for certain applications.
More recently, it was discovered that boron nitride could be produced by the pyrolysis of precursor polymers.
The advantage of this "polymer" method primarily resides in the form of the final product, and, more particularly, enables the production, after pyrolysis, of boron nitride fibers.
Thus, in U.S. Pat. No. 4,581,468 a preceramic organoboron polymer is described which is prepared by the interaction of ammonia (ammonolysis) with B-trichloro-N-tris(trialkylsilyl)borazine (a cyclic compound) and which, as indicated, after drawing and pyrolysis at 970.degree. C., results in the production of boron nitride fibers.
However, the cyclic polymer described in this patent is very difficult to prepare and is thus expensive. Therefore, it is not suitable for production on an industrial scale.
On the other hand, the maximum yield by weight of boron nitride that can be produced from such type of starting material does not exceed 22%, indicating that actual yields are on average well below this value.